The light produced by a liquid crystal display backlight is natural light, which consists of P-polarized light and S-polarized light which are perpendicular to each other in the polarization direction. The natural light is generally coupled by a light guide plate and reaches a polarizer, however, usually less than half of the light energy emitted by the backlight passes through the liquid crystal display and at least half of the light energy is consumed because only the polarized light with the polarization direction parallel to the light-passing direction of the polarizer passes through the polarizer while the polarized light with the polarization direction perpendicular to the light-passing direction of the polarizer is totally absorbed and consumed. Therefore, the light emitted by the backlight cannot be fully utilized, and the utilization rate of the light is low.